The present invention relates to an improved design for deepwater offshore platforms. More particularly, the present invention relates to an improved compliant tower design.
Compliant tower designs were developed for deepwater hydrocarbon prospects as an alternative to traditional bottom-founded platforms having fixed or rigid tower structures. While such rigid structures are effective to support topside facilities in relatively shallow to mid-depth waters, their underlying design premises become economically unattractive in developments much deeper than 1000 feet or so. Compliant towers were developed to provide bottom-founded structures in deeper water which are designed to "give" in a controlled manner in response to dynamic environmental loads rather than rigidly resist those forces. A basic requirement in controlling this response is to produce a structure having harmonic frequencies or natural periods that avoid those encountered in nature, e.g., waves. This has produced designs which, when compared with rigid platforms, substantially reduce the total amount of steel required to support topside facilities. Various approaches to altering the frequency response characteristics of such designs that have been proposed have often sought to further reduce loads and steel requirements with tightly constructed "slim" towers.
Nevertheless, these applications require great amounts of steel, and often a high percentage of this steel must be selected from premium grades and alloys. Thus, there remains substantial benefit to be gained from improvements that would safely further reduce this requirement for the amount of steel or beneficially alter the performance characteristics demanded of the steel supplied.